Summary: The intraperitoneal LD50 of Krebs 2 (K-2) and Sarcoma 180 (Sa-180) ascites tumors for inbred A2G mice was below 100 cells/mouse. Mice that had been inoculated with 106 Ehrlich ascites (EA) cells and 4 to 6 days later with WSA influenza virus underwent oncolysis and were subsequently resistant to challenge with all three tumors. This resistance could be transferred to normal A2G mice with serum or peritoneal exudate. The levels of active and passive immunity were comparable when measured with either EA or Sa 180 challenges, but definitely ower when measured with K-2. Postoncolytic EA-immune sera from A2G mice agglutinated saline cell suspensions from all three tumors to similar titers. In addition, certain sera obtained after various challenge schedules gave a precipitation line in double diffusion tests with aqueous extracts from the three tumors. These sera and tumor extracts could also be used to induce passive cutaneous anaphylaxis in guinea pigs.